1. Field of the Invention
The invention relates to geological exploration techniques, and more specifically to estimation of fluid properties from acoustic data.
2. Description of the Related Art
In various industrial processes that involve fluid material, it is useful to know the properties of the fluids involved. These properties include, for example, density, compressibility and acoustic impedance. In many applications, such as oil exploration and production, fluid properties are of particular interest. Reservoir engineers need to know the equation of state (EOS) for downhole fluids, especially hydrocarbon fluids, to decide the optimum way to produce a reservoir. An equation of state is a thermodynamic equation that relates a fluid's pressure, volume, and temperature. The simplest equation of state is the well-known equation, PV=nRT, for an ideal gas. However, the ideal gas equation is not useful downhole because, at downhole pressures and temperatures, even pure methane gas is far from being an ideal gas. The equations of state for liquids are much more complicated and often semi-empirical. There are “PVT” laboratories, which specialize in performing pressure, volume, and temperature analyses on recovered hydrocarbon fluid samples.
Attempts to correlate various physical properties of fluids to acoustic measurements are known. However, none of these other downhole acoustic methods make acoustic measurements at a plurality of pressures or attempt to determine a fluid's EOS or its EOS parameters (virial coefficients) to estimate fluid properties. For example, in U.S. Pat. No. 6,957,700, issued Oct. 25, 2005 and entitled “Self-Calibrated Ultrasonic Method of In-Situ Measurement of Borehole Fluid Acoustic Properties”, tools and methods are disclosed that determine the acoustic impedance of drilling fluid using reflections from a precise metal disk. It does not describe making acoustic measurements while the fluid pressure is changing to determine an EOS or other fluid properties.
An example, which does not describe downhole use, is provided in U.S. Pat. No. 6,763,698, issued Jul. 20, 2004 and entitled “Self Calibrating System and Technique for Ultrasonic Determination of Fluid Properties.” In this patent, a system and technique for determining fluid properties includes an ultrasonic transducer on a first surface of a solid member. A longitudinal ultrasonic pulse is delivered through the solid member and a multiplicity of pulse echoes caused by reflections of the ultrasonic pulse between the solid-fluid interface and the transducer-solid interface are detected and processed. The speed of ultrasound in the fluid is determined and the fluid density is determined as a function of the speed of ultrasound and the determined acoustic property. It does not describe making acoustic measurements while the fluid pressure is changing in order to determine an EOS or other fluid properties.
Comprehensive analysis of subsurface samples is frequently completed at a surface PVT laboratory after the sample tanks are withdrawn from a wellbore. Given that sampling may occur at depths where the environment is at a comparatively high temperature and pressure, it is recognized that the delicate balance exists for dissolved components within a retained sample. That is why a sample may be changed quite substantially by the mere act of withdrawal to the surface (where temperatures and pressure are substantially lower). Asphaltenes and waxes may precipitate out of solution and it could take weeks of agitation in the lab at high temperature and pressure (an arduous task called “recombination”) to get those components back into solution. Accordingly, various sampling techniques have included certain protocols to overcome such problems and preserve sample integrity. One such technique involves overpressuring a sample (typically by several thousand psi above formation pressure) within a sample chamber to limit or prevent separation into two phases or precipitation of certain components within the sample as the sample shrinks from cooling during its return to the surface. However, when possible, it is still preferable, to make these fluid property measurements in situ as described in the present invention. Making the measurement downhole insures that the fluid sample is in a relatively pristine state. Also, for any single run of the tool there are a limited number of sample tanks that can deployed. To test the fluids from more zones in the well than one has sample tanks requires making a downhole measurement or making a return trip into the well with the tool.
Present techniques for using acoustic signals to determine or estimate physical and chemical properties of a sample taken in the wellbore fail to provide for certain desired in-situ analyses of subsurface samples. More specifically, present techniques that employ acoustic signals for sample analysis do not make acoustic measurements at a plurality of pressures. Also, they do not use the change in acoustic properties with pressure to estimate an EOS or other fluid properties.
It is known that applying pressure to a fluid will change its acoustic properties and that one can glean additional fluid property information from how rapidly these acoustic properties change with pressure. For example, reference may be had to a journal article entitled “Non-linear Ultrasonics to Determine Molecular Properties of Pure Liquids,” Sehgal, C. M., Ultrasonics Vol. 33, No. 2, 1995, pp 155-161. This article states that wave propagation through condensed media is fundamentally non-linear, and presents several relationships between fluid properties and the rate of change in the speed of sound with pressure.
What is needed is a technique to evaluate a formation fluid sample in situ that provides some of the analyses previously only available from a surface PVT lab.